1. Field of the Invention
The present invention relates to an antiparallel pinned spin valve read head wherein magnetic and anisotropic magnetoresistive (AMR) influences on the free layer of an antiparallel pinned spin valve are balanced so as to promote high magnetic stability of the free layer.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic fields from moving magnetic media, such as a magnetic disk or a magnetic tape. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer, and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned 90.degree. to the magnetization of the free layer and the magnetization of the free layer is free to respond to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons are scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layers are antiparallel, scattering is maximized. Changes in the scattering of the conduction electrons changes the resistance of the spin valve sensor in proportion to sin .theta., where .theta. is the angle between the magnetizations of the pinned and free layers. A spin valve sensor has a very high magnetoresistive (MR) coefficient, substantially higher than an anisotropic magnetoresistive (AMR) sensor. For this reason it is sometimes referred to as a giant magnetoresistive (GMR) sensor.
A read head employing a spin valve sensor (hereinafter referred to as a "spin valve read head") may be combined with an inductive write head to form a combined magnetic head. The combined magnetic head may have the structure of either a merged head, or a piggyback head. In a merged head a single layer serves as a shield for the read head and as a first pole piece for the write head. A piggyback head has a separate layer which serves as the first pole piece for the write head. In a magnetic disk drive an air bearing surface (ABS) of the combined magnetic head is supported adjacent a rotating disk to write information on or read information from the disk. Information is written to the rotating disk by magnetic fields which fringe across a gap between the first and second pole pieces of the write head. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
An improved spin valve, which is referred to hereinafter as antiparallel pinned (AP) spin valve, is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein. The AP spin valve differs from the spin valve described above in that the pinned layer comprises multiple thin films, hereinafter referred to as AP pinned layer. The AP pinned layer has a nonmagnetic spacer film which is sandwiched between first and second ferromagnetic thin films. The first thin film, which may comprise several thin films, is immediately adjacent to the antiferromagnetic layer and is exchange-coupled thereto, with its magnetic moment directed in a first direction. The second thin film is immediately adjacent to the free layer and is exchange-coupled to the first thin film by the minimal thickness (in the order of 6 .ANG.) of the spacer film between the first and second thin films. The magnetic moment of the second thin film is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first film. The magnetic moments of the first and second films subtractively combine to provide a net moment of the AP pinned layer. The direction of the net moment is determined by the thicker of the first and second thin films. The thicknesses of the first and second thin films are chosen so that the net moment is small. A small net moment equates to a small demagnetization (demag) field from the AP pinned layer. Since the antiferromagnetic exchange coupling is inversely proportional to the net moment, this results in a large exchange coupling.
A large exchange coupling promotes higher thermal stability of the head. When the head encounters high heat conditions due to electrostatic discharge from an object, or due to contacting an asperity on the magnetic disk, a critical high temperature of the antiferromagnetic layer, hereinafter referred to as blocking temperature, can be exceeded, causing it to lose its directed magnetic moment. The magnetic moment of the AP pinned layer is then no longer pinned in the desired direction. In this regard, significant advantages of the AP pinned spin valve over the typical single film pinned layer are a greater exchange coupling field and a lower demag field, which enhance thermal stability of the spin valve sensor.
In the prior art, free layers have been constructed with thicknesses of 50 .ANG. to optimize the giant magnetoresistive (GMR) coefficient of the spin valve sensor. It would be desirable if this thickness could be increased to about 100 .ANG. for the purpose of enhancing manufacturing yield with only about a 20% sacrifice in the GMR coefficient. A 100 .ANG. thick free layer, however, can increase an AMR effect (ratio of change in resistance of the free layer to resistance of the free layer .DELTA.r/R) on the spin valve sensor to almost 1%. Unfortunately, the AMR effect seriously affects the position of the bias point of the spin valve head relative to positive and negative readback signals detected by the spin valve head, the bias point being a point on a transfer curve (readback signal of the spin valve sensor versus applied signal from the magnetic disk) of the spin valve sensor which will be described in more detail hereinafter. The AMR effect is employed in the aforementioned AMR sensor for detecting signals and is due to a change in resistance of an MR stripe in response to magnetic fields from a rotating disk. The free layer in a spin valve sensor demonstrates this same AMR effect which must be dealt with in establishing the bias point.
The transfer curve for a spin valve sensor is linear and is defined by sin .theta. where .theta. is the angle between the directions of the magnetic moments of the free and pinned layers. With positive and negative magnetic fields from a moving magnetic disk, which are typically chosen to be equal in magnitude, it is desirable that positive and negative changes in the GMR of the spin valve read head above and below a bias point on the transfer curve of the sensor be equal so that the positive and negative readback signals are equal. When the direction of the magnetic moment of the free layer is substantially parallel to the ABS and the direction of the magnetic moment of the pinned layer is perpendicular to the ABS in a quiescent state (no signal from the magnetic disk) the positive and negative readback signals should be equal when sensing positive and negative fields that are equal from the magnetic disk.
As indicated hereinabove it is desirable that the transfer curve for the sensor be symmetrically located with respect to the bias point for producing symmetrical positive and negative readback signals. This means that the bias point should be located midway between the top and bottom of the transfer curve. The designer strives to improve asymmetry of the readback signals as much as practical with the goal being symmetry. When the readback signals are asymmetrical, signal output and dynamic range of the sensor are reduced. Asymmetry improvement is easier to achieve with spin valve sensors that do not employ an AP pinned layer. Unfortunately, the free layer of the AP pinned spin valve has additional forces acting on it that displace the transfer curve relative to the bias point. These forces are addressed in the next paragraph.
The location of the transfer curve relative to the bias point is influenced by four major forces on the free layer, namely a ferromagnetic coupling field H.sub.FC between the pinned layer and the free layer, a demag field H.sub.demag on the free layer from the pinned layer, a sense current field H.sub.SC from all conductive layers of the spin valve except the free layer and the aforementioned influence of the AMR. The influence of the AMR on the bias point is the same as a magnetic influence thereon and can be defined in terms of magnitude and direction and is referred to herein as the AMR EFFECT. There is a need to deal with all of these forces on the free layer so as to improve asymmetry of the readback signals.